1. Field of the Invention
This invention relates generally to frequency divider circuits, and relates more particularly to a regenerative frequency divider having Darlington-connected transistors.
2. Description of the Relevant Art
A regenerative frequency divider includes four functional elements: a mixer, a filter, an amplifier, and a feedback network, some of which may be combined into a single physical element. The mixer, filter, and amplifier are coupled between an input port and an output port, while the feedback network is coupled between the output port of the amplifier and an input port of the mixer. An input signal having a frequency f is supplied to the mixer, which mixes the input signal with a signal from the feedback network and generates the corresponding frequency products. The output of the mixer is filtered and amplified to generate an output signal, which is also fed back through the feedback network to the mixer.
If a signal having a frequency of f/2 exists in the feedback loop, then the frequency products generated by the mixer will include the frequencies f/2, f, 3f/2, etc. The higher harmonics are filtered out by the filter and the remaining f/2 signal is amplified by the amplifier and fed back to the mixer through the feedback network. If the open-loop gain at the frequency of the f/2 subharmonic is greater than 1.0 when the input signal is applied, then the f/2 signal will be sustained in the loop. The frequency divider thus generates an output signal at one-half the frequency of the input signal.
Frequency division by integers other than 2, such as 3 or 4, is also possible, provided that the input signal is strong enough and the output frequency is within the operational band of the divider. For example, a divider having a nominal input frequency of 5.0 GHz for divide by 2 operation, will have a nominal input frequency of 7.5 GHz for operation as a divide by 3 circuit and a nominal input frequency of 10.0 GHz for operation as a divide by 4 circuit. As the divide number increases, the input signal bandwidth for proper operation at that divide number will typically decrease.
Prior implementations of a regenerative frequency divider have used either discrete or integrated components. Some prior implementations have used a single silicon bipolar transistor or GaAs field-effect transistor as a mixing/filtering/amplifying element of the divider.